The present invention relates to antennas and more particularly to a novel short length whip antenna configuration having increased gain over more than a 2.5:1 bandwidth without requiring broadband resistive loading or narrow band tuning.
It is well known that a quarter-wavelength antenna is a narrow bandwidth device, having increasing VSWR and decreasing gain as the transmission frequency is removed from the design frequency at which the antenna is exactly one quarter-wavelength long. A desirable vehicle-mounted radio-frequency antenna is characterized by operation over more than a 2.5:1 bandwidth with a substantially constant gain closely approaching that of the standard quarter-wavelength antenna. It is also desirable to have as short an antenna length as possible to minimize mechanical resonance and interference of a vehicle-mounted antenna with overhanging obstacles.
One whip antenna having a broadband, i.e., at least an octave, bandwidth is described in my copending U.S. Pat. application Ser. No. 557,836, filed Mar. 12, 1975, now U.S. Pat. No. 3,950,757. This broadband whip antenna utilizes and further requires resistive loading selectively positioned along the length of the whip antenna, which loading tends to reduce the amount of radio frequency energy radiated by the antenna, even though maintaining a low VSWR over the entire 2.5:1 bandwidth. It is desirable to eliminate the resistive loading and to further reduce the required whip length while increasing the antenna gain over more than a 2.5:1 radio-frequency bandwidth to be approximately equal to the gain of a narrow bandwidth quarter-wave antenna.